Some pundits are calling them the HillPublicans. They are hardcore Republicans who are going against their previous political beliefs and voting for Democratic presidential hopeful Hillary Clinton.

In one of the most surprising twists of an already fascinating Democratic nomination race the emergence of the HillPublicans now has many political observers poring over poll data and wondering what is going on.

One answer can be found on America's hugely influential conservative talk radio networks. A number of right-wing talk show hosts have recently been urging their solidly Republican listeners to go out and vote in the Democratic race and cast in their lot with Clinton in order to try to sabotage the Democratic Party race.

Their logic is simple enough. With the Republican nomination sewn up by Senator John McCain, their aim is to prolong the fierce battle between Clinton and Senator Barack Obama. Their hope is for a lengthy blood-letting between the two bitter rivals that will damage whoever wins the fight and leave them weakened when they go up against McCain.

America's top right-wing 'shock jock' Rush Limbaugh has been leading the charge. 'We want all the disruption in that party possible. It is about us winning,' he told his listeners recently. Other shock jocks have followed suit, telling Republicans in states still to hold their primaries to jump in and vote for Clinton.

Limbaugh has admitted that it is a tough task to persuade his fans to vote for a candidate that he has built a whole political career around viciously attacking. 'I've got a big challenge here to try to get Republicans to change their minds on this and vote for Hillary to keep her in the race, to keep that party at war with itself,' he confessed.

The exact impact of the HillPublicans and the 'Limbaugh effect' is hard to define precisely in recent races and the contests still to come. They can only have an effect in elections that are 'open' - where Republicans can opt to go and vote in the Democratic race and eschew their vote in the Republican ballot. But in recent key Clinton victories in Ohio and Texas some Republican commentators have hailed the strategy as a success. In those states exit polls showed some 9 per cent of voters were Republicans. That number is roughly twice as large as in most previous contests. They were also split almost evenly between Obama and Clinton, although Clinton has traditionally had a much more difficult time in appealing to such crossover voters than Obama. Therefore the 50-50 divide could easily show the Limbaugh effect in action. If so, the actions of the HillPublicans could have greatly helped Clinton. Her stunning comeback victories in Texas and Ohio saved her political life as her candidacy had seemingly been on the brink of political extinction.

In the most recent state to vote - Mississippi - the impact of the Limbaugh effect seemed clearer. One analysis claimed a full 25 per cent of Clinton's voters were Republicans. Though Clinton still lost the heavily black state to Obama, the margin of that defeat could have been much wider without the Republican vote. One estimate put the number of HillPublicans in Mississippi at 40,000 voters.

Clinton's campaign has dismissed the phenomenon. It claims that the large numbers of Republicans voting for Clinton - and also for Obama - show the party's crossover appeal and also reveal an electorate deeply dissatisfied with Republican rule in the White House.

But the fierce debate over the HillPublicans goes to several issues at the heart of the historical fight for the Democratic nomination. The first is that the battle seems set to stretch right through the last primary in June and could even go to the August convention in Denver. Many Republicans believe that such a fight - one that is getting increasingly bitter - has only one real winner: McCain.

The other key question that the HillPublicans pose is who Republicans believe will be the stronger opponent in the November presidential election. Many Republican strategists believe that going up against Clinton would be an easier task due to her high negative ratings and the Republican attack machine's long history of targeting Clinton.

'With Hillary Clinton there are just so many possible targets. I am sure that Republicans would be licking their lips at the thought of having her in their sights again,' said Professor Bruce Gronbeck, a political scientist at the University of Iowa.